Let Afghanistan's neighbors worry about it
All of Afghanistan's neighbors, except Iran, are not Muslim fundamentalist and have experienced Muslim fundamentalist terrorism or rebel movements. They all have in interest in stopping Muslim fundamentalism. Russia, China and India are Great Powers, none is Muslim. Pakistan is Muslim, but not fundamentalist, and is much more powerful than Afg.. These countries also have centuries of experience dealing with Afg.
I'm no expert, but maybe we should let them worry about Afghanistan. They are probably not going to let it run wild. If they did Muslim terrorists from Afg. would probably attack them long before the United States.
Iran is Muslim fundamentalist, but it is already far more of a threat than Afghanistan could ever be- it is bigger, has more people (83 million compared to 38 M), is richer, has more technology, in a more strategic location. Afg. has no flying air force now that American technicians have left. Iran has an air force. Iran has built drones and medium range missiles. Iran is working on getting nuclear weapons. Compared to Iran, Afghanistan is not that important.
map: Wikipedia
Central Asia Terrorism: National Counterterrorism Center | Groups
Comments